Instead of using GIT_SHA1_HEXSZ, use parse_oid_hex to compute a pointer
and use that in comparisons.  This is both simpler to read and works
independent of the hash length.  Update references to SHA-1 in the same
function to refer to object IDs instead.
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>